Low pressure die casting apparatus

ABSTRACT

A method of low pressure die casting in which molten metal is forced upwardly from a furnace into a die through a riser tube extending below the level of the molten metal in the furnace by gas pressure applied to the surface of the metal. To avoid the formation of oxides in the metal, an inert gas, heavier than air, is introduced into the riser tube adjacent the die to fill the riser tube above the level of the metal in the furnace. On application of gas pressure to the molten metal in the furnace, as the metal rises in the tube, the inert gas expels out from the die cavity. After solidification of the metal in the die and removal of the gas pressure to the furnace, excess molten metal falls back by gravity into the furnace, the inert gas again filling the space above the metal in the tube to prevent ingress of air at the interface between the sprue orifice of the die and the riser tube. A low pressure die casting apparatus is disclosed which includes an apertured cap on the riser tube for engagement with the sprue orifice, the cap being provided with an inlet for an inert gas.

United States Patent [191 Bloomfield et a1.

[11] 3,823,760 [451 July16,1974

LOW PRESSURE DIE CASTING APPARATUS Soag Machinery Ltd., Brentford,England Filed: Oct. 12, 1972 Appl. No.: 296,810

Assignee:

Foreign Application Priority Data Oct. 18, 1971 US. Cl 164/113, 164/306Int. Cl B22d 17/14 Field of Search 164/113, 119, 120, 66,

Galloway... 164/119 X Strom 164/62 Chatourel 164/119 X Great Britain48387/71 Ho y 164/67 Primary Examiner-.1. Spencer Overholser AssistantExaminerJohn S. Brown 57] ABSTRACT A method of low pressure die castingin which molten metal is forced upwardly from a furnace into a diethrough a riser tube extending below the level of the molten metal inthe furnace by gas pressure applied to the surface of the metal. Toavoid the formation of oxides in the metal, an inert gas, heavier thanair, is introduced into the riser tube adjacent the die to fill theriser tube above the level of the metal in the furnace. On applicationof gas pressure to the molten metal in the furnace, as the metal risesin the tube, the inert gas expels out from the die cavity. Aftersolidification of the metal in the die and removal of the gas pressureto the furnace, excess molten metal falls back by gravity into thefurnace, the inert gas again filling the space above the metal in thetube to prevent ingress of air at the interface between the sprueorifice of. the die and the riser tube. A low pressure die castingapparatus is disclosed which includes an apertured cap on the riser tubefor engagement with the sprue orifice, the cap being provided with aninlet for an inert gas.

1 Claim, 1 Drawing Figure LOW PRESSURE DIE CASTING APPARATUS BACKGROUNDOF THE INVENTION This invention relates to a low pressure die castingmethod and apparatus therefor. I r

In a low pressure die present operation, molten metal is forced upwardlyfrom a furnace into a die through a riser tube extending below the levelof the molten metal in the furnace. The molten metal is fed to the dieby applying gas pressure to the surface of the molten metal in thefurnace; the molten metal rising up the riser tube into the die wherethe metal solidifies The gas pressure is then reduced or removedallowing excess molten metal to return via the riser tube to the furnacefollowing which the die is opened and the casting removed from the die.

Problems encountered in low pressure die casting are caused by thegeneration of oxides in the metal due to air pressurisation, turbulenceduring the pouring of molten metal into the crucible and theintroduction of air when the casting is removed from the die. All thesefactors cause adulterationof the metal; the structure of the metal inthe crucible being modified and the unwanted, detrimental metal oxidesbeing contained in the finished casting.

SUMMARY OF THE INVENTION An object of the present invention is toovercome the disadvantages encountered with present low pressure diecasting methods by preventing the generation of metal oxides within theriser tube thereby minimising oxide contamination in the final casting.

A further'object of the present invention is to improve the suitabilityof low pressure die casting methods for light metals and alloys such asmagnesium and magnesium alloys.

With the foregoing and other objects in view, the present inventionconsists in a method of low pressure die casting, in which molten metalfrom a closed heated container is conveyed to the die through a risertube by gas pressure applied to the surface of the metal in saidcontainer, one end of which tube extends below the level of the moltenmetal and the other end of which is in contact relationship with thesprue orifice of the die, wherein an inert gas is introduced into theriser tube adjacent said other end of the riser tube whereby as themolten metal rises up the tube air contained in the die cavity isexpelled through vents in the die during charging of the die and as themetal in the riser tube returns to the container on removal of the gaspressure applied to the surface of the metal said inert gassubstantially prevents the ingress of air at the interface between saidother end of the riser tube and the sprue orifice and fills the spaceabove the metal in the tube.

The invention also consists in a low pressure die casting apparatus,comprising a furnace for containing molten metal, a die mounted over thefurnace, a sprue orifice in said die, a riser tube having one endextending below the level of the molten metal in the furnace and itsother end aligned with the sprue orifice in liquid tight relationship,means for applying gas pressure to the surface of the molten metal inthe furnace to force the metal through said tube into the die via thesprue orifice, and means for introducing an inert gas into said tubeadjacent said other end of the tube.

BRIEF DESCRIPTION OF TIIE DRAWING In order that the invention may bemore readily understood, reference is made to the accompanying drawingwhich illustrates diagrammatically and by way of example, one embodimentin axial section through the upper part of a low pressure die castingapparatus.

DETAILED DESCRIPTION OF THE DRAWING Referring to the drawing, the die isformed in two halves, the top half 2 of the die being attached to amovable top platen 1 and the bottom half 3 being se cured to astationary bottom platen 4 and defining with the top half 2 a die cavity5 of the die. The configuration of the die allows vents 6 to be providedin the mating surfaces between the top and bottom halves 2, 3 of thedie.

Molten metal 8 in a furnace 9 having a lid 10 is conveyed into the diecavity 5 through a riser tube 11 by gas pressure, introduced through aninlet 12 controlled by a valve 13, acting on the'surface 14 of themolten metal in the furnace 9. The lower end of the riser tube 11extends below the surface 14 of the molten metal in the furnace 9 whilstthe upper end of the riser tube 11 has an apertured riser cap 15 whichin use is aligned in liquid tight relationship coaxially with the sprueorifice 7 A flexible sealing means in the form of a bellows assembly 16of heat resistant material is located between the lid 10 of the furnace9 and a collar 17 of the riser cap 15. The bellows assembly 16 isconcentric with the riser tube 11 and may be compressed or expanded bywedge means 18 for sealing the interior of the furnace 9 and theexterior of the upper part of the tube 11 from the ambient. Theexpansion of the bellows assembly 16 exerts a force on the collar 17 toraise the riser tube 11 and press the upper surface of the riser cap 15against the sprue orifice 7 to form a liquid tight seal between theriser cap 15 and the sprue orifice 7. Other means may be employed forconnecting the riser tube 11 to the sprue orifice 7 omitting the bellowsassembly and the wedge actuating mechanism completely.

An inlet 1? is provided in the riser cap 15 for introducing an inertgas, heavier than air, into the riser tube 11. Into the aperture 20 thecap 15 there is mounted a nozzle member 21 which extends axially intothe riser tube below the level of the inlet 19. The inlet 19 iscontrolled by a valve 22.

In operation, inert gas, heavier than air, is continuously introduced,at low pressure into the inlet 19 and fills the cavity within the risertube 11 above the level 14 of the molten metal 8. The amount of gasintroduced is sufficient to make up the leakage through the die vents 6but not sufficient to cause any turbulence in the riser tube 11. Whenair under pressure is supplied to the furnace 10 via valve 13 and inlet12, the molten metal 8 is forced up the riser tube 11 pushing the inertgas before it into the die cavity 5 and flushing out air contained inthe die cavity, which is vented to the atmosphere through the vents 6.As the metal rises in the tube 11 and passes the nozzle member 21 a backpressure is produced in the cavity 23 formed between the internal wallof the riser cap and the exterior wall of the nozzle member 21 which maybe used to shut off or reduce the flow of inert gas whilst maintaining acushion of gas in the cavity 23.

When solidification of the metal in the die cavity 5 occurs, thepressure of air to the furnace is shut off and excess molten metalreturns under gravity down the riser tube 11 into the furnace. As themolten metal clears the lower end of the nozzle member 22 the backpressure exerted on the inert gas is removed and the flow of inert gasis restarted immediately substantially to prevent ingress of air intothe riser tube. Thus the formation of oxides on the exposed surface ofthe metal in the riser tube is prevented since the tube is filled withthe gas.

To remove the casting, the upper die 2 is raised thus providing accessfor air into the sprue orifice 7. However, the ingress of air isprevented from entering the riser tube by the inert gas filling theriser tube above the level of the molten metal therein. When the castinghas been removed from the die, the upper die 2 is replaced and thecasting operation repeated.

If desired, a control system is incorporated in the system to coordinatethe introduction of the inert gas with furnace pressurisation. Whenpressure is introduced into the furnace 9, a signal is'transmitted fromcontrol valve 13 to control valve 22 to initiate the flow of inert gasthrough inlet 19 to scavenge the die cavity 5. After the die cavity 5has been-filled with molten metal, a time delay is introduced to allowsufficient time for solidification of the metal casting. During thistime valve 13 is closed and the furnace vented to permit the excessmolten metal in the riser tube 11 to fall by gravity back into thefurnace, the inert gas meanwhile continuing to be introduced to fill thespace above the metal in the tube. After a further time delay, the upperplaten 1 is raised to allow removal of the upper die 2 and the casting.At this point in the cycle, a signal is sent from the platen l to thecontrol valve 22 to actuate the valve 22 to shut-off the supply of inertgas. Normally, valve 22 can only be reopened in response to furthersignal from the control valve 13. However, in order to meet thesituation which prevails at the beginning of a casting cycle, anoverride switch 22a is provided to allow valve ll to be openedindependently of control valve 13 when the riser tube 12 is not filledwith inert gas, but after the initial operation, the valve 22 iscontrolled solely by valve 13. The control signals may be electricalsignals or fluid pressure signals.

We claim:

1. A method of low pressure die casting, in which molten metal from aclosed heated container is conveyed to the die by means of gas pressureapplied to the surface of the metal in said container, through a risertube one end of which extends below the level of the molten metal andthe other end of which has an inlet nozzle member in contactrelationship with the sprue orifice of the die, said inlet nozzle memberextending axially into the riser tube below an inlet for an inert gaswhich is introduced into said other end of the riser tube adjacent saidinlet nozzle member to fill the riser. tube with said inert gas abovethe level of the molten metal when the gas pressure applied to thesurface of the molten metal is removed, wherein as the molten metalrises up the tube expelling said inert gas through vents in the die aback pressure is formed between said inlet nozzle member and said inertgas inlet to maintain a cushion of inert gas in said other end of theriser tube adjacent said inert gas inlet to prevent ingress of moltenmetal into said inlet.

1. A method of low pressure die casting, in which molten metal from aclosed heated container is conveyed to the die by means of gas pressureapplied to the surface of the metal in said container, through a risertube one end of which extends below the level of the molten metal andthe other end of which has an inlet nozzle member in contactrelationship with the sprue orifice of the die, said inlet nozzle memberextending axially into the riser tube below an inlet for an inert gaswhich is introduced into said other end of the riser tube adjacent saidinlet nozzle member to fill the riser tube with said inert gas above thelevel of the molten metal when the gas pressure applied to the surfaceof the molten metal is removed, wherein as the molten metal rises up thetube expelling said inert gas through vents in the die a back pressureis formed between said inlet nozzle member and said inert gas inlet tomaintain a cushion of inert gas in said other end of the riser tubeadjacent said inert gas inlet to prevent ingress of molten metal intosaid inlet.